You’ve seen Leonard Fournette bull over a 215-pound safety and then drag two linebackers 5 yards.

You’ve noticed him out-race a pair of defensive backs weighing 40 pounds less than him.

You’ve witnessed him leap from the 5-yard line, over a 4-foot pile of bodies, and into the end zone.

You’ve watched him leave a linebacker tumbling to the ground after one of his quick cuts, and you saw a cornerback grasp air after one of his 360-degree spins.

LSU’s Heisman Trophy frontrunner did all of this, and you probably asked yourself the obvious: “How in the heck did he do that?”

“Speed and size,” said John Brenkus, host of ESPN’s Sport Science, which explores science and physics in athletics.

That combination is the easy explanation. It’s the short answer. It’s the simple way of uncovering the big mystery surrounding No. 7 and his plethora of mind-boggling accomplishments this season.

As No. 4 LSU (7-0, 4-0 Southeastern) prepares to play at No. 7 Alabama (7-1, 4-1) on Nov. 7, Fournette has 1,352 rushing yards. That’s the ninth-most in NCAA history in the first seven games of a season, and it’s just 335 yards shy of breaking LSU’s single-season record.

His 15 touchdowns are five from cracking the school’s single-season record, and his three straight 200-yard games earlier this year were a first for any SEC player.

These numbers shock or surprise many. Not Alabama coach Nick Saban, who vigorously recruited Fournette out of high school at St. Augustine.

“We thought he was absolutely the best ... one of the best running backs we’ve ever seen,” Saban said. “He certainly hasn’t done anything to disappoint us in terms of what he’s been able to accomplish as a college player and maybe one of the most dominant guys in the country relative to what he does.”

Big and fast

The average college running back stands 5-foot-11 and weighs 213 pounds. Fournette is 6-1 and 230.

That’s big.

Fournette’s top-end speed has been clocked at 20.5 mph. That’s better than the top-end speed of Seattle Seahawks defensive back Richard Sherman, Brenkus said.

That’s fast.

It’s that combination of big and fast that sets Fournette apart.

“It’s rare,” said Brenkus, who has studied NFL and college running backs for years. “In terms of what we’re seeing, he’s breaking new ground. It’s Herschel Walker-esque, but he’s doing things Herschel didn’t do.”

INTERACTIVE: Hover your cursor over the image to learn more about Leonard Fournette's freakish skillset.

All quotes and numbers from Jon Brenkus of ESPN's Sports Science | Advocate staff photo by Bill Feig

Brenkus has studied film of Walker, the Heisman-winning running back out of Georgia. Because of his size — he played at nearly the same height and weight as Fournette — Walker is the best comparison for Fournette.

Fournette’s speed-and-size combo results in what you’ve seen so many times this season: No. 7 running over defenders or carrying them 3 yards.

Fournette at full speed creates the same momentum as a 500-pound Bengal Tiger running at 10 mph, Brenkus said.

“His size and speed is creating the same force as a tiger coming at you,” Brenkus said. “It is like being in a car crash, that’s for certain. That’s literal. You’re experiencing ... over 2,000 pounds of force. The moment of impact, over a ton of force.”

No wonder Texas A&M’s Howard Matthews and Auburn’s Blake Countess plummeted to the turf on collisions with Fournette. They ran into what Brenkus called “the perfect storm.”

“You can say Fournette is the perfect storm for a running back. He’s big, but not too big. He’s fast, but not too fast,” he said. “His speed is elite, but it’s not so fast that the force he’s experiencing is too much for his body. That’s what happens to smaller running backs. He’s in the Goldilocks zone.”

Fournette has broken more than 60 tackles through seven games this season. Brenkus has the answer why.

“It’s because his speed is faster than a normal running back and weight is heavier than a normal running back,” he said. “The amount of force is going to be higher. It takes more to stop him.”

In the blink of an eye

Some things about Fournette can’t be explained by science.

“He did a pirouette when we were playing John Curtis,” said Cyril Crutchfield, Fournette’s high school coach at St. Aug. “He did this pirouette in the air. He’s just a natural.

“Leonard, he’s so instinctive. He relies on instincts. Either you have it or you don’t. He has it.”

That might be Fournette’s most overlooked and under-appreciated attribute. Fournette’s decisions during runs aren’t easy.

Should I use the spin move against this defender, or slow up and stutter-step? Do I follow the fullback to the left, or cut right? Should I lower a shoulder, or leap into the air?

These decisions have to be made quickly. How quickly? Within one-tenth to four-tenths of a second.

“These decisions that he’s making are literally at the speed of a blink of an eye,” Brenkus said.

In the time it took you to read that quote from Brenkus, Fournette could have made about four of those decisions. He also could have made four or more cut moves.

Fournette pushes off the ground with four times his body weight to make cuts in two-tenths of a second. Brenkus said that cut speed is on par with those measured from Minnesota Vikings running back Adrian Peterson, the 2012 NFL MVP.

Peterson is about 5 to 10 pounds lighter than Fournette.

How he does it is somewhat inexplicable. How can he be so big but so fast and agile?

“God-given ability,” Crutchfield said.

Brenkus credited Fournette’s “elite vision.” He sees the field better than most.

Saban went back to instincts. He called Fournette an “instinctive” runner who understands the play, reads blocks and “maximizes the efficiency of that particular play.”

“He does (that) about as well as anybody,” the coach said.

A good core

As a former offensive lineman, Shawn Elliott likes the proposition of blocking for Leonard Fournette.

Why?

“It doesn’t have to be blocked perfectly,” said Elliott, the former South Carolina assistant promoted to head coach when Steve Spurrier resigned a few weeks ago. “He can take on open-field tackles, safeties or a corner, and he can shred them like no other.”

Part of that shredding involves Fournette’s vicious, sudden cuts. But what happens after the cuts? Fournette doesn’t lose as much speed as you might think.

Fournette can make a cut of 60 degrees and retain 85 percent of his speed, Brenkus said. So what does that mean exactly?

If Fournette were at midfield running at 20 mph toward the north end zone of Tiger Stadium and made a change in direction toward the LSU sideline, his speed would drop only to about 17 mph.

“That’s a difficult thing to wrap your mind around,” Brenkus said. “It’s rare for somebody that big to be able to do it. Smaller running backs can do that, but bigger running backs cannot.”

At least, they usually can’t.

Fournette’s muscular core is behind his speed retaining power, Brenkus said. It’s also behind those 360-degree spins.

Fournette is able, because of his muscle structure, to control his “center mass” better than others. This brings balance to his frame, and it helps his velocity overall, too.

“Think about it,” Brenkus said. “You can be that big but not be able to generate enough muscle mass to create enough energy to propel yourself forward at a quicker rate. Larger athletes generally aren’t that fast, because you need to create enough energy to propel yourself forward.”

Earlier this season in a win over Auburn, Fournette showed off all of his attributes. He stutter-stepped to juke a defensive end. He darted toward the sideline, maintaining that speed, to whiz by a linebacker. And then he bashed into a defensive back.

And then he bashed into another defensive back.

And then he bashed into another defensive back.

Agility. Speed. Size.

The third defensive back wrestled Fournette out of bounds. The running back looked toward the heavens and gave a head-shaking roar.

There are some things even science can’t explain.

“He’s one of a kind,” Crutchfield said.

MORE LEONARD FOURNETTE COVERAGE

Follow Ross Dellenger on Twitter: @DellengerAdv.